In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a “drill string”, then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. A typical drill string is made up from an assembly of drill pipe sections connected end-to-end, plus a “bottomhole assembly” (“BHA”) disposed between the bottom of the drill pipe sections and the drill bit. The BHA is typically made up of sub-components such as drill collars, stabilizers, reamers and/or other drilling tools and accessories, selected to suit the particular requirements of the well being drilled.
In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel). During the drilling process, a drilling fluid (commonly referred to as “drilling mud” or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annular space (“wellbore annulus”) between the drill string and the wellbore. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall (to stabilize and seal the borehole wall), as well as other beneficial functions.
As an alternative to rotation by a rotary table or a top drive, a drill bit can also be rotated using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill string immediately above the drill bit. The technique of drilling by rotating the drill bit with a mud motor without rotating the drill string is commonly referred to as “slide” drilling. It is common in certain types of well-drilling operations to use both slide drilling and drill string rotation, at different stages of the operation.
One of the primary components of a downhole motor is the power section, which is commonly provided in the form of a progressive cavity motor (or “PC motor”) comprising an elongate and generally cylindrical stator plus an elongate rotor which is eccentrically rotatable within the stator. As is well known in the art, a PC motor is essentially the same thing as a positive displacement pump (or “Moineau pump”), but operated in reverse, and therefore could also be referred to as a positive displacement motor. Although all of these terms thus may be used interchangeably, for simplicity and consistency the term “PC motor” will used throughout this patent document.
The rotor of the PC motor is formed with one or more helical vanes or lobes encircling a central shaft and extending along its length. The stator defines helical lobes of a configuration generally complementary to the rotor lobes, but numbering one more than the number of rotor lobes. In the typical operation of a downhole motor, drilling fluid flowing downward through the drill pipe assembly is diverted through the PC motor, causing the rotor to rotate within the stator, thus rotating a drive shaft and resulting in rotation of the drill bit (which is operably connected to the drive shaft through other components of the downhole motor and BHA).
A vertical wellbore (i.e., a wellbore that is intended to be vertical) can deviate the desired vertical path during the drilling process by reason of the drill bit deflecting when encountering subsurface obstacles such as faults or discontinuities in the formation through which the well is being drilled. Such deviations must be corrected in order for the wellbore to achieve the desired end point, and it is known to correct a deviated wellbore path using an orientable steerable downhole motor in conjunction with directional drilling techniques. However, the wellbore may deviate from the desired corrective path when using a steerable downhole motor due to difficulty with controlling the orientation of the drill string and the necessity of using slide drilling techniques with this drill string configuration. Accordingly, there is a need for simpler, more reliable, and less expensive systems and associated control mechanisms for driving and steering rotating downhole tools to return a deviated vertical wellbore to a vertical path.
A directional wellbore (i.e., a wellbore or a portion of a wellbore that is intended to have a non-vertical path) requires steering during the drilling process to have the resulting wellbore reach the predetermined target. Known directional drilling techniques using an orientable, steerable downhole motor are commonly used to direct the wellbore along a desired three-dimensional path, and to correct wellbore path deviations caused by subsurface obstacles and irregularities. However, as in the previously-discussed case of deviated vertical wellbores, the use of an orientable, steerable downhole motor to correct deviated directional wellbores can be complicated or frustrated by difficulties controlling the orientation of the drill string and the necessity of using slide drilling techniques with this drill string configuration. Accordingly, there is a further need for simpler, more reliable, and less expensive systems and associated control mechanisms for driving and steering rotating downhole tools to return a deviated directional wellbore to the intended path.